An abbreviation for "fake mets fan" or "fake jets fan."
This term is usually used when a fan to either the mets or the jets does something un-fan-like such as not knowing a player, not knowing a team fact or missing a game, etc...

"Yoni, your such a fets fan. How could you have missed that game?"
"How do you not know who the running back is Elliot? Your such a fets fan."